In digital subscriber line (DSL) systems, available frequencies for transmission may be divided into frequency bands, which may be referred to as carriers or tones. The division may be done through discrete multi-tone (DMT) modulation.
In DSL systems, each tone is allocated a different number of bits. For example, for each tone i, a different number of bits bi is transmitted. The number of bits for each tone is stored in a Bit Allocation Table. For the tone i, 2bi quadrature amplitude modulation (QAM) modulation is used. Consequently, the number of bits in each tone dictates a size of a QAM constellation for the tone.
The amount of data that can be transmitted across a tone may vary depending upon the quality of the signal at a frequency range associated with the tone.
Moreover, the number of bits in each tone can be reallocated through bit swapping or Seamless Rate Adaptation (SRA). Bits are allocated to tones based on the measured signal to interference plus noise ratios (SINRs) of the tones, respectively. For example, a number of bits in a first tone may be decreased if the SINR of the first tone is below a SINR margin. To offset the decrease, a number of bits in a second tone may be increased if the SINR of the second tone is above the SINR margin. This requires that bit changing be continuously performed based on changing SINRs.